The present invention is directed to the field of beamforming, particularly as used with an adaptive antenna array for a wireless telecommunications system, e.g. a wireless local area network (WLAN). In previous-type WLAN systems, it had been sufficient to communicate with wireless clients using one or more omnidirectional antennas. In such a previous-type scheme, wireless clients gain access to the WLAN by operating on different frequency bands and/or time-sharing over the same set of frequency bands.
As the number of clients in a WLAN increases, with resulting increased demands for WLAN access, it becomes necessary to “manage space,” i.e. spatially isolate communications between clients distributed over a geographic area. To this end, it has become common to employ a directional antenna that can be selectively pointed at clients to allow isolated communications between the clients and the WLAN.
A common implementation for a directional antenna is to use an adaptive array. Such arrays can be formed of any grouping of antenna elements, such as a dipole, Yagi and patch antennas. These arrays can be one-dimensional, i.e. having linearly-distributed antenna elements. The array can also be two-dimensional, i.e. spread over an area, or three dimensional, i.e. distributed within a volume. Another common type of antenna is a printed array formed by lithographic techniques.
As the number of clients in a network continues to increase, it becomes increasingly hard to avoid interference between wireless clients, even when using an adaptive antenna array. Also, multipath interference can result from reflections and/or diffraction of the client' signal off metal within the building in which the WLAN operates. For reducing interference, it is possible to provide a narrow beam that can be steered toward a desired client using an array. Alternatively, it is possible to steer a “null” toward a potential interference source, where a “null” is an angular distribution in the array antenna pattern of very low gain signal strength.
In practice, it is difficult and expensive to form a narrow beam, requiring adaptive arrays with more elements and a high level of precise calibration. However, such arrays are undesirably expensive, due to the level of testing and calibration. Also, with potential sales volumes of several hundred thousand antenna arrays per year, such handling slows down production in addition to adding to the expense, thereby further reducing production efficiency.
Without calibration and testing, presently available lithographic techniques allow the construction of printed arrays having great precision, having a tolerance of +/−0.003″. An error of 0.005″ in a printed array has been found to produce a small wavelength error of only 0.2% at the 5.0 GHz band. Thus, an array as manufactured would have very desirable performance, except for the expense accounted in testing and calibration.